Numerous industrial processes use acids in their processing that contaminate the acid with process by-products in waste. These contaminated acids are commonly referred to as “spent acid”. Industrial chemical and petroleum processes are prime examples. Many of these processes require costly processing of the process acid to remove impurities. For example, acids are widely used in industrial chemical and petroleum refining applications that require acid “regeneration,” which generally means removal of contaminants (including often water) from the process acid to restore the acid to, or near to, its original process specification or requirements. Handling spent acid also raises safety and environmental concerns.
An exemplary acid use to illustrate the present invention is sulfuric acid, which is used in a number of petrochemical and petroleum refining processes. Depending on the use, commercial “fresh” acid is typically supplied in strengths of 70-99.5 wt % sulfuric acid with the remainder typically being water. Many uses generate a waste or spent acid stream containing organic hydrocarbon materials. This spent acid stream is typically reprocessed to remove the organic material. Incineration and reconstitution of the sulfuric acid is conventionally used to remove the organic material.
An exemplary petroleum processing use of sulfuric acid is as a catalyst for alkylation processes. In a typical alkylation process the relatively high purity, concentrated sulfuric acid becomes diluted or contaminated with water and organic hydrocarbon materials commonly referred to as acid soluble oil (ASO). When sufficiently diluted or contaminated, the catalytic activity of the acid degrades. Spent sulfuric acid from the alkylation process can be regenerated but at a considerable cost using existing methods.
Conventional methods for spent acid regeneration involve generally combustion of the spent acid to form sulfur dioxide, conversion of the sulfur dioxide to sulfur trioxide, and absorption of the sulfur trioxide in water. While this technology is widely used to produce high strength acid (>98 wt % H2SO4), it is capital intensive. Thus, it is usually more economical for low volume users of sulfuric acid to transfer spent sulfuric acid by various means such as rail, truck, ship, or pipeline to a central regeneration facility rather than construct their own facilities for acid regeneration. Freight costs can be a significant part of the total costs for regenerating spent acid.
Sulfuric acid can also be concentrated from about 70 wt % H2SO4 to about 85 wt % or about 96 wt % sulfuric acid by using evaporation with one or two stages. The evaporation method is highly energy intensive as the acid/water mixture must be heated to a high temperature to vaporize the water. It also requires special materials such as glass lined vessels and tantalum heaters to prevent corrosion. An improved, less expensive method for regenerating spent sulfuric acid is needed.
Accordingly, there is ample need for a separation process to efficiently and effectively remove impurities from process contaminated acids to restore the acid to or near its original process specification. It would also be beneficial if that process could be deployed “in situ” with the process that produced the spent acid.